Magnetic resonance imaging (MRI) is an emerging technology used for detection of disease and for follow-up diagnosis after surgery or treatment. While MRI shows great potential because of its high spatial resolution, deep soft tissue contrast, and use of non-ionizing radiation, its low sensitivity remains a drawback. To overcome this shortcoming, paramagnetic contrast agents, such as Magnevist® (an FDA approved chelated gadolinium reagent), can be used to enhance the detection sensitivity of MRI. The conjugation of contrast agents to a macromolecular platform further enhances imaging sensitivity. Reduced molecular tumbling rates of gadolinium ions after conjugation to such nanoparticles results in increased longitudinal relaxivities. Waters, E. A. & Wickline, S. A., Basic Research in Cardiology, 103, 114-121 (2008). Furthermore, multivalent display results in increased local concentration, both events are contributing to increased sensitivity. Various nanoparticle systems have been explored as supramolecular contrast agents; these include dendrimers, liposomes, perfluorocarbons, silica, as well as protein cages and virus-based nanoparticles, also termed viral nanoparticles (VNPs). Bruckman, M, Steinmetz, N, “Engineering Gd-loaded nanoparticles to enhance MRI sensitivity via T1 shortening,” Nanotechnology, 24, —————— (2013).
VNPs, specifically plant viruses and bacteriophages, have received tremendous attention in recent years. They have been developed as research tools and platforms for materials science as well as for potential nanomedical applications. Lee et al., Biotechnol Bioeng. 109, 16-30 (2012). The propensity to self-assemble around a cargo (the genome) and to deliver this cargo to specific cells and tissues, make viruses ideal candidates for site-specific delivery of therapeutics and/or contrast agents. Indeed, several VNP-based technologies are in clinical testing for gene delivery and oncolytic virotherapy. VNPs are attractive materials because of their high degree of symmetry, polyvalency, monodispersity, and their genetic or chemical programmability. Most VNP structures have been solved to atomic resolution, which allows tailoring with a high degree of spatial control. Using chemoselective bioconjugation reactions, VNPs can be modified with imaging contrast agents, therapeutic moieties, and/or targeting ligands such as peptides or antibodies. For example, preclinical imaging of prostate tumors has been demonstrated using cowpea mosaic virus (CPMV) modified with prostate cancer-specific peptide ligands (bombesin) and near infrared imaging dyes. Steinmetz et al., Small 7, 1664-1672 (2011). Moving toward translational research, several research groups have engineering VNPs with paramagnetic MRI contrast agents. Similar to other nanoparticles, increased relaxivities are achieved based on reduced tumbling rate of the contrast agent. Huang et al., Theranostics 2, 86-102 (2012). For example, bacteriophage MS2, a 27 nm sphere, was loaded with ˜180 chelated Gd molecules using a TOPO ligand and was able to achieve ionic relaxivities of up to 41.2 mM−1s−1 per Gd ion and 7,416 mM−1s−1 per nanoparticle. In comparison, Magnevist® has a relaxivity 5.2 mM−1s−1. Anderson et al. Nano Letters 6, 1160-1164 (2006); Garimella et al., JACS 133, 14704-14709 (2011).
To date, research and development of VNP-based MRI contrast agents has focused on spherical platforms; however, this may not be optimal. Recent work by the inventors and others indicates improved pharmacokinetics, increased immune evasion (e.g., reduced macrophage uptake), increased tumor homing, tissue penetration, and vessel wall targeting of elongated particles, e.g. potato virus X and tobacco mosaic virus Shukla et al. Molecular pharmaceutics, 10(1):33-42 (2013), Lee et al. Biomaterials Science 1, 581-588 (2013). Wen et al. Biological Physics, 39(2):301-25(2013). However, the use of rod-shaped virus particles as imaging agents remains unexplored.